1. Field of the Invention
The present invention relates to a communication system. More particularly, the present invention relates to an apparatus and method for receiving data in an uplink (UL) of a Broadband Wireless Access (BWA) communication system.
2. Description of the Related Art
In the next generation communication systems, active research is underway with the goal of providing users with high-speed services having various Qualities of Services (QoSs). Active research is especially underway with the goal of guaranteeing mobility and QoS for Broadband Wireless Access (BWA) communication systems, such as a Wireless Local Area Network (WLAN) system and a Wireless Metropolitan Area Network (WMAN) system. Representatives of such next generation communication systems include an Institute of Electrical and Electronics Engineers (IEEE) 802.16a/d communication system and an IEEE 802.16e communication system.
The IEEE 802.16a/d communication system and the IEEE 802.16e communication system, which are BWA communication systems, employ an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme in order to support a broadband transmission network for a physical channel of the WMAN system. The IEEE 802.16a/d communication system considers only a single cell structure with a state in which a current Subscriber Station (SS) is stationary, without considering mobility of the SS at all. In contrast, the IEEE 802.16e communication system considers mobility of the SS in addition to considerations made by the IEEE 802.16a communication system. As used herein, an SS having mobility is called a Mobile Station (MS).
According to the OFDM/OFDMA scheme, sub-carriers within one OFDM symbol are divided so as to be used by multiple users (i.e. multiple MSs). A communication system based on the OFDM/OFDMA scheme includes separate physical channels for transmitting control information. Uplink fast feedback information, which is a type of control information as described above, includes Signal to Noise Ratio (SNR), differential SNR of each band, fast Multiple Input Multiple Output (MIMO) feedback, mode selection feedback, etc. Further, the uplink control information of the communication system, that is, feedback information transmitted from each of the MSs to a Base Station (BS) that is providing communication service to the MS, includes Channel Quality Information (CQI) between the MS and the BS and information of Acknowledgement (ACK)/Non-Acknowledgement (NACK) in response to data transmitted by the BS. Specifically, the feedback information includes ACK when the MS has successfully received data transmitted from the BS to the MS, and includes NACK when the MS has failed to receive the data. That is, the MS feeds back an ACK message to the BS when it has successfully received data transmitted from the BS, and feeds back a NACK message to the BS when it has failed to receive the data.
In such a communication system, the quantity of the uplink ACK information is low in consideration of the entire communication service. In other words, the data transmitted from the MS to the BS does not include a large quantity of ACK information. For example, the data includes only one bit of uplink ACK information. And although the quantity of uplink ACK information is relatively small, it is very important in management of a communication system, and requires a high reliability in transmission thereof. However, in order to reduce the overhead ratio, a physical channel for transmission of the uplink ACK information is typically not allocated a large quantity of frequency-time resources. Therefore, regarding the physical channel used for transmitting the uplink ACK information, it may be advantageous to use a transmission scheme different from a scheme for transmitting a large quantity of information.
In general, in order to transmit uplink control information, the communication system uses a method of combining binary channel coding with coherent modulation or differential modulation.
However, when the uplink ACK channel is transmitted by using a small quantity of frequency-time resources according to the above-mentioned method, the probability of error increases and the stability of the communication system decreases. In contrast to the case in which there are sufficient pilot tones for transmission of available uplink traffic areas or downlink areas, the pilot tones for transmission of uplink ACK information may become insufficient. Therefore, the channel estimation performance decreases and the performance of the coherent modulation/demodulation scheme also decreases. At this time, if the number of pilot tones is increased in consideration of only the channel estimation performance, the number of data tones may become insufficient. Further, separation of the binary channel code and the modulation may decrease the optimized performance. Moreover, if a large quantity of frequency-time resources are used in transmitting uplink ACK information in order to enhance stability, the overhead ratio increases and the throughput of the communication system decreases.
Therefore, in order to achieve reliable and effective data transmission/reception in a communication system, a scheme capable of enabling a BS to detect exact uplink ACK/NACK information is necessary. In other words, in a communication system, when data to be transmitted to an MS occurs, the BS performs scheduling for transmission of the data, and then transmits the data to the MS through a downlink channel based on the scheduling. Then, as described above, the MS transmits feedback information including the ACK information to the BS in response to the data transmitted from the BS, and the BS having received the feedback information determines whether to retransmit the data in accordance with the ACK information. At this time, in the case of retransmitting the data, the BS again performs scheduling for the data retransmission and then retransmits the data. In this process, in order to enhance the data rate, throughput of the data and the resource use efficiency, a scheme for receiving and detecting the feedback information of the MS, for example the ACK information, is necessary.